Maybe we might see Hilbert appear in Pokémon X and Hilda in Pokémon Y as a cameo as being one of the most strongest trainers in the game, while they'll act the same way as what Red did, by not saying anything.

'Cause as we know, in Pokémon Black Version 2 and Pokémon White Version 2, Hilbert/Hilda's mother said that the character went far away in search for N. If Hilbert and Hilda made a return, then it surely would be interesting where it would end up being.

As for other cameo appearances where no battles are involved, maybe we might see Nate and Rosa as well, depending on which version you're playing.